1. Field of the Invention
The present invention relates generally to a heat pipe, and more specifically to a semi-loop heat pipe having co-current, swirling two phase flow in the evaporator, and an impermeable return line from the condenser.
2. Description of the Prior Art
Heat pipes are devices that employ the evaporation and condensation of a working fluid contained within to effect the transfer of energy from the evaporator where heat is absorbed to the condenser where the heat is released. Heat pipes gained prominence in the early 1960's as superconducting, heat transfer devices as detailed, for example, in U.S. Pat. Nos. 3,229,759 and 4,485,670. While numerous configurations and applications of heat pipes have been proposed since their initial invention, the basic heat pipe is still viewed as a unit that can transport large quantities of energy over a relatively small temperature gradient.
Heat pipes are containment vessels that are charged with a working substance which is continuously evaporated and condensed as heat is added to the evaporator and removed from the condenser. The rate at which vapor is produced is directly proportional to the rate of heat flowing into the heat pipe. The ability of a heat pipe to efficiently transfer energy rests on the fact that non-condensable gaseous species within the chamber are removed from the heat pipe prior to operation. As such, a heat pipe is evacuated prior to its use as a heat transfer device. By eliminating non-condensable gases from the chamber, the vapor that is generated in the evaporator flows to the condenser down a pressure gradient in much the same way as a pump causes fluid to move through an enclosure. With the presence of non-condensable gases, the vaporized working substance would move by molecular diffusion down a concentration gradient. Given that a pressure driven flow can be orders of magnitude more effective in moving vaporized working substance, heat pipe systems are generally evacuated. Conversely, if the heat pipe chamber develops a leak, the heat pipe will cease to function. Thus, the use of a heat pipe in a high temperature environment can be problematic if the evaporator experiences insufficient cooling as this can cause the containment vessel to be perforated with the subsequent failure of the heat pipe.
Heat pipes can generally be classified into two main categories, namely, those wherein the vapor and liquid flow countercurrent to each other, and those wherein the liquid and vapor flow in a co-current manner. Countercurrent flow heat pipes are well known in the prior art. FIG. 1 shows a simple countercurrent heat pipe, where the vapor flow rises through the center from the evaporator at the bottom, is condensed in the upper portion and flows as liquid down the sides to the liquid pool in the evaporator. Their operation is well described by Grover in U.S. Pat. No. 3,229,759, and by Camarda et al. in U.S. Pat. No. 4,485,670. The combination of gravity and capillary forces generated within a wick on the interior walls of the heat pipe are used to return liquid working substance to the evaporator from the condenser.
Co-current heat pipes are generally referred to as loop heat pipes, examples of which are disclosed in U.S. Pat. Nos. 4,515,209 and 5,911,272, depicted respectively in FIG. 2 and FIG. 3. Both co-current and countercurrent heat pipes often contain a wick on the inner evaporator surface to ensure uniform coverage by utilizing the capillary forces generated by the wick to spread the liquid.
While both loop and non-loop (i.e. countercurrent) heat pipes have been used in a number of products and applications, they have not been incorporated in units where high heat fluxes at high operating temperatures are encountered and they are generally not used in large scale units. This is largely because such systems are amenable to failure of the containment material that forms the heat pipe. In order to ensure that the containment vessel has durability and a long life, it is necessary to have the entire evaporator of the heat pipe unit adequately cooled by the working substance in the unit. This has not been possible as yet with the heat pipes of the prior art.
Thus, insufficient cooling of even a relatively small region (e.g. 10 mm2) can lead to the perforation and subsequent destruction of the heat pipe unit. Heat pipes of the prior art have rarely been intended for use in applications involving high operating temperatures, and as such, destruction of a heat pipe chamber as a result of exposure to elevated temperatures has never been adequately addressed.
A controllable heat pipe is described in U.S. Pat. No. 5,159,972 comprising a reservoir for the liquid and a separate return line to the top of the evaporator, as shown in FIG. 4. However, this heat pipe nevertheless fails to overcome the principle difficulties associated with all countercurrent heat pipes used in high heat flux applications.
The three main limitations of prior art heat pipes that must be overcome to make their use in high temperature applications feasible are: film boiling on the evaporator walls, levitation of the liquid returning to the evaporator, and configurational complexity of a loop heat pipe for certain applications.
The levitation of liquid from the leading end of the evaporator will reduce heat transfer efficiency and will, if the temperatures are high enough, cause the heat pipe to fail as a result of dry-out. The levitation of liquid is of greatest concern in large scale units where the length of the evaporator can be sizeable. In such units the refluxing of liquid down to the bottom of the evaporator can be a major concern because the total heat load on the unit can be large even if the heat flux is moderate. Since the heat load manifests itself as a vapor flow, the vapor velocity at the top of the evaporator of a large scale unit can be enough to create some degree of fluidization of the liquid.
The other principle difficulty with using heat pipes in high heat flux applications is the onset of film boiling on the evaporator walls. As is well known to those skilled in the art, this can reduce the rate of heat extraction by as much as an order of magnitude. This dramatically reduces the heat transfer efficiency and, in some cases, may lead to the destruction of the evaporator containment walls.
One possible use for heat pipes is in a reagent delivery unit such as a lance. U.S. Pat. No. 5,310,966 describes a heat pipe lance, or tuyere. However, the heat pipe lance of U.S. Pat. No. 5,310,966 fails to teach how to eliminate the levitation of liquid from the leading end of the evaporator or how to eliminate the formation of a stable vapor film on the inner walls of the evaporator.
Loop heat pipes can overcome the issue of entrainment, however, loop heat pipes are often not viable for many practical applications because of their configurational complexity, wherein the return loop pipe is run outside the main heat pipe body which significantly increases space requirements of the heat pipe. Nevertheless, as with countercurrent heat pipes, the problem of film boiling on the evaporator surfaces nevertheless remains.
The mechanism for evaporation remains an important limiting factor in a heat pipe, and especially for high heat flux applications. If the working substance is of low thermal conductivity and the heat flux is relatively high, the working substance will experience boiling at the interface between the liquid and the heat source. If the generation of vapor is sufficiently intense, a stable vapor film will ultimately form between the liquid phase of the working fluid and the evaporator wall. This vapor film will greatly inhibit heat transfer. The evaporator has then attained its boiling limit, and the subsequent result of continued exposure to the heat flux can be overheating of the evaporator walls and possible failure of the heat pipe.